Method for electrolytic production of refractory metal



y 1960 w. GENDVIL ET AL 2,936,269

METHOD FOR ELECTROLYTIC PRODUCTION OF REFRACTORY METAL Filed Oct. 18, 1956 HEAT/1V6 EZECTEOOE A EU/VA H0 W 554 0 V/L FL/VE/F W M0455 6 H0 wA/w R PAM/EH A as/vr United States Patent METHOD FOR ELECTRQLYTIC PRODUCTION OF REFRACTORY METAL Leonard W. Gendvil, Dongan Hills, N.Y., Oliver W.

Moles, Plainfield, NJ., and Howard R. Palmer, Heuderson, Nev., assignors to National Lead (lompany, New York, N.Y., a corporation of New Jersey Application October 18, 1956, Serial No. 616,699

4 Claims. (Cl. 204-64) The present invention relates to the production of a refractory metal and more especially to the electrolytic production of titanium metal.

Various electrolytic methods are known forproducing titanium metal and in general these methods may be diferentiated as cells of the type having a diaphragm to prevent halogenation of the titanium ions at the anode; and diaphragmless type cells wherein titanium tetrachloride is introduced into the electrolyte in immediate proximity to a cathodic surface, the current to titanium tetrachloride feed rate being regulated so that titanium ions are reduced substantially immediately to titanium metal on the cathode, thereby preventing migration of the titanium ions to the anode.

Titanium metal of good quality can be produced by both of these types of electrolytic cells provided, however, suflicient care is taken to prevent contamination of the titanium metal by deleterious elements or compounds and in particular oxygen. Although some appreciation of the high aifinity of oxygen for titanium metal was had early in the development of electrolytic cells for the production of titanium metal, it was not until much time and labor had been expended in the design, construction and operation of electrolytic cells that the full import of this problem was realized. Experimental evidence has been obtained showing that to insure the production of titanium metal of high ductility, no more than and preferably less than, 0.05% oxygen may be tolerated in the titanium metal product. At the outset extreme care was exercised in the selection of the refractory materials and cements used in the construction of the cells not only to seal the cells from the atmosphere but to keep to a minimum the amount of oxygen introduced into the electrolyte from the refractory materials. Subsequently, it was found that a major source of contamination had its origin in any oxygen in the atmosphere above the electrolyte. Apparently any oxygen in the atmosphere above the surface of the electrolyte reacts with any reducing agents present at the surface of the electrolyte to form metallic oxides which enter the electrolyte and contaminate the titanium meal product.

An object of the invention is to operate an electrolytic cell for producing a refractory metal in such a manner as to preclude the contamination of the refractory metal by oxygen and other deleterious elements.

A further object of the present invention is to provide an improved process for producing titanium metal electrolytically wherein an electrolytic cell is operated in a manner such as to maintain the atmosphere above the electrolyte substantially free of oxygen.

A still further object of the invention is to produce substantially pure titanium metal electrolytically by maintaininan atmosphere of chlorine above the surface of the electrol te in the cathode area of an electrolytic cell.

These and other objects of the instant invention will become apparent from the following more complete description and the accompanying drawings in which the single figure is a vertical section of a diaphragmless type 7 cell for carrying out the process of the present invention.

In its broadest aspects, the instant invention relates to a process for producing a refractory metal in the form of relatively large coarse crystals of high purity byproviding a fused salt electrolyte in an electrolytic cell having an anode and a cathode, providing reduced refractory metal halide values in the electrolyte, passing current through said electrolyte and preventing reoxidation at the anode of the reduced refractory metal values in said electrolyte while continuously maintaining a protective atmosphere above the surface of the electrolyte, thereby to preclude oxygen contamination of the refractory metal depsit on the cathode.

Due to the presence in a diaphragm cell of reduced refractory metal chloride values throughout the catholyte and the possible preferential atfinity of chlorine for these values with the consequence of reoxidation of the reduced refractory metal chloride values in the catholyte, the applicability of the instant invention to a diaphragm type cell would be limited. However, the invention is especially effective with the diaphragmless type of electrolytic cell hereinabove mentioned.

As used herein, the term refractory metals will be understood to include such metals as titanium, zirconium, titanium, hafnium and tantalum, and for purposes of illustration only, the following description will relate specifically to the production of titanium metal by the process of the instant invention which, of course, is applicable to the production of any of the refractory metals identified above.

Referring to the drawing, the figure shows a typical dia phragmless type cell which comprises a covered container 32, substantially rectangular in cross-section sealed from the atmosphere and filled or partially filled with a fused salt electrolyte 13, in which is suspended a graphite anode 14, and a metal basket-type cathode 15, having a tubular feed pipe 16- for introducing gaseous titanium tetrachloride into the electrolyte in immediate proximity to a cathodic surface of the cathodic basket.

As is the practice in the art, the fused salt electrolyte 13 is preferably a molten mixture of a halide salt of an alkali or alkaline earth metal including magnesium and particularly the chlorides; and the process is carried out by passing a predetermined amount of current through the electrolyte at a rate synchronized with the rate of tetrachloride addition such that the titanium values in the bath are reduced to titanium metal on the cathode before having a chance to reach the anode. in this connection a theoretically sufiicient current would comprise about 4 faradays of electricity passed through the electrolyte While introducing l mole of titanium tetrachloride. However, in actual practice it has been found desirable to add a quantity of electricity somewhat in excess of the theoretical amount in order to make up the current loss caused by side reactions in the cell. With the type of cell shown in the figure it has been found desirable to add from about 4.5 to 6.0 faradays of electricity per mole of titanium tetrachloride introduced. Other operational factors such as the cathode current density and the character of the cathodic surface are described in more detail in US. Patent No. 2,749,295.

In operating the diaphragmless type of cell described above, current is passed through the electrolyte to reduce the titanium values in the electrolyte to titanium metal which is deposited on the cathode in the form of large coarse crystals. It has been found necessary to the production of a pure highly ductile titanium metal to protect the titanium metal deposit from contamination by oxygen in the atmosphere above the electrolyte; and this may be accomplished in a practical and economical manner by introducing a protective gas atmosphere into the cell above the surface of the electrolyte and in particular 3 t that part of the cell in which the titanium metal deposit is being produced. In the case of the diaphragrnless cell of the figure, the protective gas extends above the entire upper surface of the electrolyte.

The function of the protective gas atmosphere is to prevent any oxygen in the atmosphere above the electrolyte from reacting with any reducing agents present at the surface of the electrolyte, thereby forming metallic oxidic compounds which enter the electrolyte and contaminate the titanium metal deposit.

In this regard it has been found that highly satisfactory results are obtained and that a purer and more ductile titanium metal product may be produced when a halogen gas and in particular chlorine is employed above the surface of the electrolyte. Moreover, it has been found that chlorine and other halogen gases are far more effective than an inert gas for preventing oxygen from entering the electrolyte, and it is believed that the superior results obtained by using chlorine gas result not only from the fact that the chlorine gas physically displaces any oxygen containing atmosphere above the electrolyte but that the chlorine reacts chemically and preferentially with any reducing agent with which it comes into contact, thereby precluding the oxygen in the atmosphere from reacting with the reducing agents. Thus, no oxides are formed that might enter the electrolyte and contaminate the titanium metal deposit.

In practice, the chlorine gas formed at the anode may be used as a source of chlorine both for physically displacing any oxygen containing atmosphere above the surface of the bath and for chemically and preferentially reacting with any reducing agents to preclude the formation of oxides. In the diaphragmless type of cell shown in the figure, the chlorine formed at the anode is allowed to flow freely across the entire upper surface of the electrolyte. By using the chlorine developed at the anode, supplemented by additional chlorine gas if needed, the efliciency of the electrolytic process for making titanium metal has been found to be unusually high and the titanium metal deposit is substantially wholly recoverable as a highly ductile commercially acceptable product.

In order to illustrate the operation of various embodiments of this invention, the following examples are given.

Example I Using a diaphragmless type cell similar to that shown in the figure having a basket type cathode and feed pipe 16 for the introduction and retention of titanium tetrachloride vapors in the electrolyte in the immediate vicinity of a cathodic surface of the basket, a fused salt electrolyte consisting of 50 pounds of sodium chloride and 1 pound of lithium chloride was provided in the cell and heated to a temperature of 900C. Chlorine gas was maintained above the surface of the electrolyte in the cathode chamber portion of the cell by passing 1 gram per minute chlorine gas over the surface of the bath. Titanium tetrachloride vapors were introduced at the rate of-2.8 grams per minute into the feed portion of the basket and electric current equivalent to 6 faradays per mole of titanium tetrachloride was concurrently passed through the cell, the current requirement being 150 amperes at an impressed voltage of approximately 10.5 volts. The cathode current density was about 0.6 ampere per square centimeter and the anode current density was about 0.3 ampere per square centimeter. The cell resistance was substantially 0.05 ohm.

.The process was continued for a period of 14 hours at which time the introduction of titanium tetrachloride vapor was stopped and no further current was passed through the cell. Titanium metal was deposited in the basket cathode in the form of an irregular tenacious mass of relatively large crystals which after leaching weighed 538 grams, analyzed 99.9% titanium and had a Brinell hardness value of 110. i I

4 Example 11 In order to demonstrate that the results achieved above by the use of chlorine as a protective atmosphere are superior to those resulting from the use of inert gases, the experiment described above was repeated using argon gas as a protective atmosphere. The yields and efficiencies were substantially the same as those obtained in Example I supra except that the quality of the titanium metal produced was inferior, the Brinell hardness of the titanium metal being 130 as against 110 in the previous example.

The above examples clearly show that a superior type of titanium metal possessing a relatively low Brinell hardness may be obtained when chlorine gas is employed to remove an oxygen containing atmosphere above the surface of the electrolyte; and that the objects of the in vention have been achieved wherein a new and improved process has been developed for producing pure ductile titanium metal.

Although the electrolytic production of titanium metal has been used .to illustrate the invention, it will be apparent that the maintenance of a chlorine atmosphere above the electrolyte to react with any reducing metals at the surface of the bath will be equally effective regardless of the type of refractory metal being produced and that, therefore, the invention is also applicable to the electrolytic production of refractory metals other than titanium.

The invention may be embodied in other specific forms without departing from the spirit and essential characteristics thereof and the specific embodiments described are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are, therefore,

intended to be embraced therein.

We claim:

1. In a method for electrolytic production of refractory metals in an electrolytic cell having an anode, a cathode and a bath of a molten salt selected from the group consisting of the chlorides, bromides and iodides of alkali metals, alkaline earth metals, magnesium and mixtures thereof wherein a halide of the refractory metal is introduced into the bath and electric current is concurrently passed through said bath between said anode and said cathode to deposit a refractory metal on said cathode, the improvement comprising: maintaining above the catholyte portion of said bath an atmosphere of a gas having preferential .afiinity for any reducing agent in lieu of oxygen or oxygen compounds at the surface of said bath both to form with said reducing agent a compound innocuous to the titanium metal in said bath and render innocuous any oxygen in the atmosphere above said bath.

2. In a method for electrolytic production of refractory metals in an electrolytic call having an anode, a cathode and a bath of a molten salt selected from the group consisting of the chlorides, bromides and iodides of alkali metals, alkaline earth metals, magnesium and mixtures thereof wherein a halide of the refractory metal is introduced into the bath and electric current is concurrently passed through said bath between said anode and said cathode to deposit a refractory metal on said cathode, the improvement comprising: maintaining above the catholyte portion of said bath an atmosphere of a halogen gas having preferential affinity for any reducing metal in lieu of oxygen or oxygen compounds at the surface of said bath, thereby to render innocuous any oxygen in the atmosphere above said bath.

3. In a method for electrolytic production of refractory metals in an electrolytic cell having an anode, a cathode and a bath of a molten salt selected from the group consisting of the chlorides, bromides and iodides of alkali metals, alkaline earth metals, magnesium and mixtures thereof wherein a halide of the refractory metal is introduced into the bath and electric current is concurrently passed through said bath between said anode and said cathode to deposit a refractory metal on said cathode,

the improvement comprising: maintaining above the catholyte portion of said bath an atmosphere of gaseous chlorine.

4. A process for producing titanium metal in an electrolytic cell having a cathode, an anode and a bath of a molten salt selected from the group consisting of the chlorides, bromides and iodides of alkali metals, alkaline earth metals, magnesium and mixtures thereof which comprises: introducing titanium tetrachloride into said bath, concurrently passing electric current through said cell at a rate such that the amount of current being addedis sufiicient to reduce the titanium tetrachloride to metal on said cathode, and maintaining a chlorine gas atmosphere over the entire surface of said bath.

2,556,763 Maddox June 12, 1951 5 2,749,295 Svanstrom et a1. June 5, 1956 2,785,066 Dean Mar. 12, 1957 FOREIGN PATENTS 10 678,807 Great Britain Sept. 10, 1952 OTHER REFERENCES The Extraction of Titanium, Worner and Cordner, Australian Institute of Mining and Metallurgy Proceedings, 15 New Series No. 158-159, 1950, page 73. 

1. IN A METHOD FOR ELECTROLYTIC PRODUCTION OF REFRACTORY METALS IN AN ELECTROLYTIC CELL HAVING AN ANODE, A CATHODE AND A BATH OF A MOLTEN SALT SELECTED FROM THE GROUP CONSISTING OF THE CHLORIDES, BROMIDES AND IODIDES OF ALKALI METALS, ALKALINE EARTH METALS, MAGNESIUM AND MIXTURES THEREOF WHEREIN A HALIDE OF THE REFRACTORY METAL IS INTRODUCED INTO THE BATH AND ELECTRIC CURRENT IS CONCURRENTLY PASSED THROUGH SAID BATH BETWEEN SAID ANODE AND SAID CATHODE TO DEPOSIT A REFRACTORY METAL ON SAID CATHODE, THE IMPROVEMENT COMPRISING: MAINTAINING ABOVE THE CATHOLYTE PORTION OF SAID BATH AN ATMOSPHERE OF A GAS HAVING PREFERENTIAL AFFINITY FOR ANY REDUCING AGENT IN LIEU OF OXYGEN OR OXYGEN COMPOUNDS AT THE SURFACE OF SAID BATH BOTH TO FORM WITH SAID REDUCING AGENT A COMPOUND INNOCUOUS TO THE TITANIUM METAL IN SAID BATH AND RENDER INNOCUOUS ANY OXYGEN IN THE ATMOSPHERE ABOVE SAID BATH. 